Controlled flux in calcining cadmium pigment crude



Feb. 7, 1950 G. c. MARcoT coNTRoLLED FLUX 1N CALCINING CADMIUM PIGMENT CRUDE 3 Sheets-Sheet l Filed June 28, 1946 ATTORN EY Feb. 7, 1950 I G. c. MARcoT 2,496,588

coNTRoLLED FLUX 1N cALcINING CADMIUM PIGMENT cRuDE Filled June 28, 194e s sheets-sheet 2 `F,. GUY c. Mn/Pcor,

ATTO R N EY 070 Pfr/ 5c 7a4/vcr Feb' 7 1950 G. c. MARcoT 2,496,588

CONTROLLED FLUX IN CALCINING CADMIUM PIGMENT CRUDE Filed June 28, 1946 s sheets-sheet 5 ATTO R N EY Patented Feb. 7, 1950 coN'rRoLLED FLUX IN oALoINING CADMIUM PIGMENT CRUDE Guy '(3. Marcot, Lynchburg, Va., assignor 'to American Cyanamid Company, New York, N. Y., corporation vof Maine Application .I une 28, 1946, ,Serial No. 680,100

This invention relates to the production of Cadmium Red pigments and is concerned "particularly with a method of preparing a composite of controlled flux content which composite on calcination yields a'pigment of optimum quality. The invention also relates to pigments so produced.

As used 'in the present invention, the term cadmium red pigments is used to designate any of a variety of calcined products ranging in color from bright orange to a Fdark maroon and containing both CdSfand CdSe. These components appear to form a solid solution in the iinished pigment, constituting the principal colored constituent thereof. Control oi the color shade is obtained by Yvarying the relative proportions of CdS and CdSe inthe solid solution, approximately as indicated in the following table:

TABLE I GdSOdSe Ratio shade Percent v Percent CdS CdSe Orange 7o 30 Light Redso 4 0 Dark Red.. v48 52 Maroon l 41 59 Deep Maroon r gwn- 35 65 While these proportions are known, various shades having been previously obtained by a number of proposedmethods, these methods have not been wholly satisfactory and are subject to a number of procedural and economic disadvan`- tages. Particularly is this true as to the production of the darker shades.

In addition to preparing these pigments as "pure types, i. e., forms in which the solid 'so-lu tion is the principal constituent, they are also commonly prepared as"f1ithopo'ne or extended types. This is usually done by including afsuit-A able amount of an inert "extender such, 'for example, as BaSO4, in the lcrude before calcination. These extenders may be incorporated in the crude in anydesired manner, usually, however, by coprecipitation with one or more of its components. The process of the present invention is readily adapted and its advantages equally applicable to the manufacture of either pure or extended forms.

In the past, cadmium red pigments, such as those with which the present invention is ooncerned, have been obtained 'by combining'a 'series of procedural steps, each in turn selected from various proposals, to produce a calc'inable crude. Such procedures may be roughly divided into t'Wo generalclassifications; precipitation oi the crude as vcompounds of 'indefinitestructure, usually designated as cadmium sulio-selenides; and

4 Claims. (Cl. V10ft-4307.1.)

2 preparation as a mechanical mixture of the components.

The sulfo-selenide type of crude ls usuallyobtained by dissolving selenium in a sodium or barium sulide liquor and carrying out a strike of'thi's liquor and a solution of a solublev cadmium salt. The resultant cadmium-bearing precipitate is used as the crude. In .preparing the vmixture type of crude= CdS is separately prepared and then combined, usually in a slurry, with a CdO reactant, which may be lCdOy per se or a compound reducible during calcination to 'Cdg and with an Se reactant, preferably elemental Se. In either type of preparation, Vsome sulfur is also usually present in the crude.

In either case, the crude to be calcined, however prepared, is collectedQdried il necessary, and linally calcined. Atsuitable temperatures, this results in vconversion of at least part ot' the vselenium and an equivalent amount of cadmium compound or compounds to Cd'Se. This reaction is accompanied by the concurrent liberation of sulfur dioxidel and also, in previously-used processes, some free sulfur which is volatile .and eliminated at the usual caloination temperatures of about W-1400o F. l y Y However, the sensitivity of the crude to jealcination `has a very marked effect von the quality of the-iinal pigment product. the present discussion, the term an insensitive crude isV used to designate a composite which on .calcination exhibits only Vminor changes in the product per unit change in the heat treatment. Asensin tive crude is used to refer toarcomposite that exhibits considerable variation in mass tonevand tinting strength for each unit change in the heat treatment. i y

Neither type is wholly desirable. 4An insensitive crude even When 4calo-inedunder optimum conditions, yields a product which isvdeiicient to standard. An overly sensitive crude, on the other hand, sinters so severely that a high quality product is never obtained. If a crudehavingthe oorrect degree of sensitivity could -be produced, it would be expected on calcination toyield anoptimum quality pigment. In the past, however, the conditions which tgovern sensitivity have Kbeen considered to .be v.too criticalr and complicated to be subject .to regulation by methods which are both simple and reliably duplicated.

t is, therefore, the object of .the present invention to .devise .a method which will enable the production of crude composites which .on .caloination meet the desired' conditions.k They must be suiciently sensitive to enable' reliable 'reproduction of the optimum mass tone and tint-` ing strength, Without being subject to excessive eintering. These objects have vbeen ound'to .be simply .and readily, obtained by insuring the presence during .calcination of na controlled opti'- rnum amount of lii'nringY salts. n f

3 While the present invention is not limited thereto, it is well illustrated in preparing cadmium red pigments in accordance with the process set forth in my copending application, Serial- No. 680,098, iiled of even date. In that application a process is set forth comprising the y calcination at from about 1U00l400 F. of a crude obtained by slurrying together individual slurries of cadmium sulfide; cadmium oxide, usually as a material reducible thereto during calcination such as cadmium carbonate; and powdered elemental selenium. The composite should contain Se and CdGO-s in such amounts that the SezCdO ratio is 3 :2, whereby the formation of CdSe during calcination Will be in accordance with the reaction vthe preferred process of the present invention is normally .by the reaction of a cadmium salt, preferably the sulfate, with a metal sulde. For example, if a pure type pigment is to he made, this is usually NazS to produce a soluble sulfate and precipitate CdS. Normally, if a slurry of this precipitate is combined directly with the other slurred components of the crude, the latter is too sensitive to calcination. This is partly due to the fine particle size of the CdS so obtained and partly to its high content of adsorbed and/or entrained soluble salts.

It would seem simple to remove these salts by washing, thereby overcoming the diiiiculty. However, any attempt to do so is physically limited. The salts appears to have a flocculating eiect. Washing is limited by the thixotropic and dispersive character of the slurry resulting from a reduction of the soluble salts content to belov7 the flocculating point. A CdS slurry which has been Washed to the optimum point imposed by these physical effects will still have a Watersoluble salt content of about 1.5-3.0%, which is too high to permit optimum calcination if carried over into the crude.

As noted above, in the preferred practice of this invention the CdS slurry usually is admixed with separately prepared CdCOs and Se slurries to produce a composite crude which is dewatered, dried and calcined. In so doing, diiiiculty with an excess of water-soluble salts in the CdS slurry is partially offset. The CdCOs seems to discharge or release a part of the Water-soluble salts from the CdS and they are removed during dewatering. Otherwise, the latter would be preferentially calcined and a good quality product would never be obtained. Nevertheless, even after such mixing and normal dewatering, the crude still contains sumcient Water-soluble salts to result in too great sensitivity during calcination.

To overcome this objectionable situation neces sitates either a further reduction in salts content or the addition thereto of an anti-luxing agent to oset their eifect on calcination. Further reduction in salts content is impractical because of the excess number of washing operations required. On the other hand, past attempts to add an anti-fluxing agent have not been productive of a satisfactory solution because of the difficulties in selecting and adding the correct amount.

In accordance with the present invention it has been found that these diilculties may be readily overcome. This is done by a combination of two factors. First, a suitable anti-fluxing agent is added at the correct point in the process to overcome the excessive fluxing effect of the normally expected salts content of crude. A suflicient amount is used to insure the crude being insensitive. Secondly, a suicient amount of iiuxing salts is subsequently added to produce a predetermined iiuxing effect.

The most desirable point of addition of the anti-iiuxing agent appears to be as an end-treatment in the preparation of the CdS. Accordingly, the anti-fluxing material is added to a slurry of the CdS precipitate. If so desired, the precipitate may be washed either before or after the material is added. While the choice of anti-uxing agents may be from a number of materials, they must be insoluble. For the purpose an insoluble phosphate is preferable. To be thoroughly disseminated in the precipitate they are preferably formed in situ. This is easily done by adding phosphoric acid to a slurry of precipitated CdS and precipitating an insoluble phosphate, either by means of (Ba) ions already present or by adding some material such as aluminum sulfate specifically for the purpose.

The amount used will depend somewhat upon the amount of soluble salts present after washing. Usually about 0.2 to about 1.0% of the Weight of the CdS, or, for a lithopone type, of the total pigment, in the slurry is normally adequate. Other factors being equal, a good general practice is to add an amount equivalent to about 0.2-0.6% by Weight of the total solids.

This treatment will account for the soluble salts introduced into the composite in the manufacture of the CdS slurry when a pure type pigment is being prepared. In making an extended type, in which the CdSOi is reacted with BaS to precipitate both CdS and CaSO'i this is not so important a consideration. The present invention therefore has a most noticeable improvement over past processes in the preparation of pure type pigments.

A second source, applicable to the manufacture of both pure and extended types, is in conjunction with the preparation of the CdO,` which as noted above is usually added in the form of CdCOa. The cadmium carbonate is ordinarily prepared by the strike of an aqueous solution of some cadmium salt, preferably the sulfate, and a solution of sodium bicarbonate. Although a large proportion of the salts introduced in this reaction may be washed out of the resultant precipitate, it is not practical to reduce the salts content to less than about 0.5% of its original content. In using a slurry of CdS, which has been end-treated with an insoluble phosphate, as set forth above, in the making of the composite this does no harm. The antifluxing material introduced in the end-treatment of the CdS precipitate is ordinarily quite adequate to oiiset an excessive iiuxing effect of the salts from this source.

From the foregoing discussion it will be seen that when using the end-treated CdS slurry the composite CdS, CdCOc and Se mixture, after normal Washing and repulping, will have a watersoluble salts content low enough so that the crude will fbe relatively insensitive to calcination. Furaanwas ther, it then can beeasilyfand directly adjusted to the proper degree of sensitivity to calcination by adjusting the total Aamount of luxing salts present-tothe correct level.

Final'adjustment of the flux content is obtained by-'a'dding predetermined amounts of substantially any vvateresoluble sodium or potassium salts. "Ihe salt should be compatible with the product-and Ytherefore the sulfate is probably preferable `for the purpose, although the invention is `not necessarily limited thereto. Some 'salts will'be present in the washed `and repulped composite, even though the vfluxing effect thereof is inadequate or odset. It has been found that a good practiceisto add sodium or potassiumsulfate to the composite in amounts approximately equivalent'to theresidual molecular equivalents of soluble'salts in the composite, calculated as sodium'sulfate.

While the `added iiuxing agent may be either the sodium or the potassium'salt, vthe potassium salt is 'preferable since this produces an approximately equimol'ecular amount of the sodium and potassium salt in the nal crude before calcination. This'appears to produce pigment development without'excessive sintering, somewhat betterthan'when either salt is present alone. The eiiect, however, is small and the invention is not limited thereto.

Actually, the amount 'of sodium or potassium salt added is usually'small. The `amount of'soluble saltsv present in 'the 'composite is readily vdetermined by pulping a measured quantity with distilled waterand testing the conductivity. It will usuallyvary from "about 0.10 'to as high as about 0.5%, normally about 0.3-0.4%. Adding an "approximately 'equal -amount will therefore produce a total of from. about 0.5% to about 1.0%. Where testing facilities are not available, a good average practice is to add about 0.3-0.5% of a potassium fsalt.

The invention Will be more fully illustrated in conjunction with 'the lfollowing examples which are intended as illustrative only and not by way of limitation. All parts are by Weight unless otherwise noted.

EXAMPLE 1 Extended Pigment-Light Red Shade A CdS litlropone slurry Was Vformed by a strike of .suiiicient .1.70 molar CdSOi liquor and 0.72' molar BaS liquor to 'precipitate 'a total of CdS-PBa'SOe of about 540'parts which was treated with a 0.03 mol 'excess of BaS' per mol of CdS, with 1.62 parts of HsPOi, an .amount about equivalent to 0.3%z of the solids weight, and ltered', the 'lter' cake being washed and repulped to a volume of about 2000 parts. A C'dCOs reactant was prepared by a strike of sufficient 1.76 molar 'CdSOil-lliquor with 1.12 molar solution of NaI-ICO: to produce 65.3 parts of CdCOa which was washed suii'lciently to reduce the soluble salts content to about 1.3%v oi the initial content and repulped to about 435 parts. A Se reactant was preparedfby pulping 45.3 parts of powdered Se metal at 50% solids with a 0.5% aqueous BaS solution. y.Ancomposite slurry was prepared by adding the Se reactant to the CdS slurry and then adding the CdCOs slurry, thoroughlymixing, the Whole and'adjusting the total volume to about 2500'- parts.

To show thefeieet :of the 'iiuxing salt control, one-half of the composite-,slurry was directly dewatered and dried at .e5-.95 C. A test by a tint tone evaluations.

6 standardized :conductivity procedure indicated that the dried'cake contained approximately 0.3% of soluble alkali metal salt, which because of the formulation used comprised lessentially NazSO4. This was designated as Sample A.

To the remaining one-half of the composite vslurry wasadded about 3.5 parts of KzSO4, .this

being approximately an equi-molecular amount to the sodium sulfate present. A conductivity test indicated about 0.6% of water-soluble salts. This slurry was then dewatered and dried at about -95" C. This cake was designated as lSample B.

.Both vSamples A and B were .calcined -in an electric muiile furnace in a non-oxidizing atmosfphere of CO2 for .15 minutes at temperatures'from an. initial 11.00 F. to a final 1200-1250 F.. the calcined pigments being quenched, filtered and dried. The dried pigments were evaluated by Inu-lling the pigment in iinseed oil ata pigment content of about '70%. Rubouts of the mixture were taken for examination of the mass tone and zinc oxide reductions prepared for examination ofthe tint tone. Both the mass tone and the tint tone oi the pigment obtained from Sample `IB were superiorto those from Sample A, the mass tone being vboth brighter and cleaner and the tint tone being appreciably cleaner.

vFor I purposes ofr comparison, reflectance curves of 'boththe mass tone and the reduced tint tone rubouts of bothsamples were taken. These'are shown in Figure 1 in which the curves designated A-l 'and A-2 are respectively the reiiectance curves'for the mass tone and tint tone vrubouts obtained from Sample A, and B-I and B-2 designate the corresponding curves of the pigment obtained from Sample B. Comparison of these curves indicates the clear lsuperiority of thepig-A mentirom Sample B.

EXAMPLE 2 lPure Type Pigment-Orange Shade 380 parts of CdS were precipitated by a simultaneous strike of 2.35 molar Cd'SO4 liquor and 1.0 molar NazS liquor, the resultant slurry being treated with a 0.05 mol per mol of CdS excess of Na2S,-With 3.8 parts of HsPOi and an equivalent quantity of AlziSOrls and filtered, the filter cake being washedfand repul-ped to .about 2000 parts. TheCdCOa reaetant was prepared as in Example l toobtain 96.parts of CdCO: slurried to a total volume. of 630 parts and the Se reactant was prepared by slurrying 66.5 parts of Se powder at 50% solids in a 01.5% aqueous BaS solution.

The Se slurry and the CdCOz slurry were-successively added to the CdS slurry and the Whole admixedand adjusted to a total volume of 3500 One-half this slurry was directly dewatered and dried, the resultant cake being desig hated as Sample C. The remainder was supplemented byradding an amount `of Na2SO4 approximately'eq-ual. to that already Vpresen-t and then dewatered Aand dried. The resultant cake was'v designated as Sample D. Sample C contained aboutv0.4% and Sample D about 0.8% respectivelyo-i water-soluble salts calculated as NazSOiV equivalents.

Sample C was calcined for 12 minutes from an initial temperature of 1100 Esto a final temperatme of 1300 F. Sample D was also calcined for i2 minutes but at temperatures from/an. initial w50" to' afnal 1250915".

.As in Example .1; the'products were assayedby preparing rub-out t'ests'in'oi'l for vmass tene and The .mass vtone "of pigment from Sample D was both cleaner and brighter than that 'from Sample C and the tint tone from Sample D was denitely superior. Spectrophotcimetric curves showing the marked distinction in the red end of the spectrum of both pigments are shown in Figure 2' in which curves C and D represent the curves obtained from the mass tone rubouts of Samples C and D respectively.

EXAMPLE 3 Pure Type-Light Red Shade Example 2 was repeated using 280 parts of CdS in a 2000 parts by volume slurry, 95.8 parts of CdCOz in a 705 parts by volume slurry, andGBA parts of Se in a 50% solids slurry, the composite slurry being adjusted to a total Volume of 3000 parts.

1500 parts of the composite slurry was directly dewatered and dried, the filter cake containing 0.3% of Water-soluble salts which was designated as Sample E, and the remaining 1500 parts having added thereto suicient KzSOi to produce a Water-soluble salts content of about 1.0% before being dewatered and dried, yielding a filter cake designated as Sample F. Sample E was calcined for 10 minutes over a temperature range from an initial of 1100" F. to a iinal of 1350" F. and Sample F being calcined for over l minutes from an initial temperature of 1000 F. to a final temperature of 1250 F. '-The resultant pigments were of a light red shade, that from Sample F being brighter and cleaner in mass tone and much cleaner in tint tone than that from Sample E. Rubouts in oil to evaluate mass tone and tint tone were made. Spectrophotcmetric curves of the mass tone rubouts are shown in Figure 2 in which curves E and F are those of the pigments obtained from Samples E and F respectively.

EXAMPLE 4 "Pure Type Pigment of Medium Red Shade Example 2 was repeated using 268 parts of CdS in a 2000 parts by volume slurry, the precipitate being treated with 0.5% by weight of H3130.; and an equivalent amount of Al2(SO4)s. 107 parts of CdCO3 were suspended in a 800 parts by volume slurry having a final salt content of 0.6% that of the initial salts. 74.4 parts of Se were formed into a. 50% solids slurry. The slurries were combined as in Example 2 and washed to a soluble salts content, as equivalents of NazSOi, of about 0.2%.

' One-half this slurry Was dewatered and dried irectly to obtain the lter cake as Sample G. The remaining portion of the slurry was adjusted to a total Water-soluble salts content of 1.0% With K2SO4 and dewatered and dried to obtain a filter cake as Sample H. These samples Were calcined over a 10 minute period, Sample G at from 1l00 lik-1350 F. and Sample H at from 1000 F.1250 F. Rubouts of the resultant medium red pigments indicated that the pigment from Sample H was both brighter and cleaner in mass tone and cleaner in tint tone than the pigments from Sample G. Spectrophotometric curves of the rubouts of both the mass tone and the zinc oxide reduced tint tone samples are shown in Figure 3 in which curves G-l and G`2 are those of the mass tone and tint tone rubouts of the pigment from Sample G and curves H--l and I-I--Z are the corresponding curves for the pigment from Sample H.

Pure Type Pigment of Maroon Shade Example 2 was repeated using 279 parts'of CdS treated with l by Weight of HsPOi and an equivalent Weight of Al2(SO4)3 in a 2000 parts by volume slurry; a 177 parts of CdCOi in a 1200 parts by volumeslurry and 123 parts of Se powder as a 50% solids pulp. The combined slurries were adjusted to a total volume of 3500 parts, 1750 parts thereof being directly deWa-tered and dried to produce Sample I and which contained 0.2% Water-soluble salts and the remaining v17.50 parts being adjusted with NazSOi to produce a lter cake containing 1.0% water-soluble.l salts as Sample J. i

Samples I and J were calcined over 10 minutes, Sample I at from 1200 F.-1400 F. and Sample J at from 1200 F.-1350 F. Rubouts of the resultant pigments which possess a maroon shade indicated that the product from Sample J was definitely superior in both mass tone and tint tone than the pigment from Sample I.

I claim:

1. In producing cadmium red pigments by calcining a mixture of a cadmium sulde reactant, powdered selenium and a cadmium oxide reactant selected from cadmium oxide and compounds easily calcinable to cadmium oxide; the improved method of controlling the fluxing properties of the mixture which comprises: forming the cadmium sulfide reactant by a strike of an aqueous cadmium salt solution and a solution of a salt selected from the group consisting of the alkali metal and barium suldes; Washing thev Watersoluble salt content of the resultant thixotropic precipitate down to the about 1.5-3.0% limit which cannot be practically reduced by Washing due to the thixotropic nature of the precipitate and supplementing the precipitate with about 0.2-1.0% of the total pigment Weight of a waterinsoluble phosphate anti-iiuxing agent; forming a slurry of the so-treated precipitate; combining that slurry with slurries of the powdered selenium and the cadmium oxide reactant, whereby a composite mixture is obtained; Washing the composite to a soluble-salt content, when dewatered, of about 0.1-0.5% of the solids; adding suilicient Water-soluble salt, selected from the group consisting of the salts of sodium, potassium and mixtures thereof, to raise the total Water-soluble salt content to about 0.5-1.5% of the solids, dev/atering and drying the resultant composite.

2. A process according to claim 1 characterized in that the anti-fluxing agent is an insoluble aluminum phosphate.

3. A process according to claim 1 in which the cadmium sulfide precipitate is Washed after adding the anti-uxing agent.

4. A process according to claim 1 in Which the iluxing salt added to the Washed composite is a sulfate.

GUY C. MARCOT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,148,194 Drucker Feb. 21, 1939 2,248,408 Juredine 1 .July 8, 1941 FOREIGN PATENTS Number Country l Date 299,797 Great Britain 1928 

1. IN PRODUCING CADMIUM RED PIGMENTS BY CALCINING A MIXTURE OF A CADMIUM SULFIDE REACTANT, POWDERED SELENIUM AND A CADMIUM OXIDE REACTANT SELECTED FROM CADMIUM OXIDE AND COMPOUNDS EASILY CALCINABLE TO CADMIUM OXIDE; THE IMPROVED METHOD OF CONTROLLING THE FLUXING PROPERTIES OF THE MIXTURE WHICH COMPRISES: FORMING THE CADMIUM SULFIDE REACTANT BY A STRIKE OF AN AQUEOUS CADMIUM SALT SOLUTION AND A SOLUTION OF A SALT SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METAL AND BARIUM SULFIDES; WASHING THE WATERSOLUBLE SALT CONTENT OF THE RESULTANT THIXOTROPIC PRECIPITATE DOWN TO THE ABOUT 1.5-3.0% LIMIT WHICH CANNOT BE PRACTICALLY REDUCED BY WASHING DUE TO THE THIXOTROPIC NATURE OF THE PRECIPITATE AND SUPPLEMENTING THE PRECIPITATE WITH ABOUT 0.2-1.0% OF THE TOTAL PIGMENT WEIGHT OF A WATERINSOLUBLE PHOSPHATE ANTI-FLUXING AGENT; FORMING A SLURRY OF THE SO-TREATED PRECIPITATE; COMBINING THAT SLURRY WITH SLURRIES OF THE POWDERED SELNIUM AND THE CADMIUM OXIDE REACTANT, WHEREBY A COMPOSITE MIXTURE IS OBTAINED; WASHING THE COMPOSITE TO A SOLUBLE-SALT CONTENT, WHEN DEWATERED, OF ABOUT 0.1-0.5% OF THE SOLIDS; ADDING SUFFICIENT WATER-SOLUBLE SALT, SELECTED FROM THE GROUP CONSISTING OF THE SALTS OF SODIUM, POTASSIUM AND MIXTURES THEREOF, TO RAISE THE TOTAL WATER-SOLUBLE SALT CONTENT TO ABOUT 0.5-1.5% OF THE SOLIDS, DEWATERING AND DRYING THE RESULTANT COMPOSITE. 